Cathode Outlet Research Articles

Power-to-methane via co-electrolysis of H2O and CO2: The effects of pressurized operation and internal methanation

This paper presents a model-based investigation to handle the fundamental issues for the design of co-electrolysis based power-to-methane at the levels of both the stack and system: the role of CO2 in co-electrolysis, the benefits of employing pressurized stack operation and the conditions of promoting internal methanation. Results show that the electrochemical reaction of co-electrolysis is dominated by H2O splitting while CO2 is converted via reverse water-gas shift reaction. Increasing CO2 feed fraction mainly enlarges the concentration and cathode-activation overpotentials. Internal methanation in the stack can be effectively promoted by pressurized operation under high reactant utilization with low current density and large stack cooling. For the operation of a single stack, methane fraction of dry gas at the cathode outlet can reach as high as 30 vol.% (at 30 bar and high flowrate of sweep gas), which is, unfortunately, not preferred for enhancing system efficiency due to the penalty from the pressurization of sweep gas. The number drops down to 15 vol.% (at 15 bar) to achieve the highest system efficiency (at 0.27 A/cm2). The internal methanation can serve as an effective internal heat source to maintain stack temperature (thus enhancing electrochemistry), particularly at a small current density. This enables the co-electrolysis based power-to-methane to achieve higher efficiency than the steam-electrolysis based (90% vs 86% on higher heating value, or 83% vs 79% on lower heating value without heat and converter losses).

The uniformity and consistency analysis of a fuel cell stack with multipoint voltage-monitoring method

The non-uniform internal status in one cell and the inconsistency between cells have significant impact on the performance and service life of commercial fuel cell stack. The identification of this phenomenon with in-situ method is the key technology for product development and optimization. This paper presents the corresponding analysis and a practical in-situ method, i.e., the multipoint voltage-monitoring method, to identify the fuel cell stack uniformity and consistency. Cross plate current and voltage variation in bipolar plate can be therefore observed using the proposed method. In addition, an one-dimensional model has been developed to explain this phenomenon. A variable loading rate experiment has been conducted in a four-cell stack to explore the relationship between consistency and uniformity in multi-cell stack. When the loading rate increases from 20 mA/cm2·s to 40 mA/cm2·s, the current concentrating in the cathode outlet also increases. Current non-uniformity is caused by gas distribution inconsistency between cells, which makes cell1 drier than the other cells. Voltage difference between the two monitors of cell1 is about 32mV, and it can be even larger in extreme conditions, like fuel starvation. It means the health condition estimation of fuel cell stack can be easily misdirected, when monitor is placed on different position. Experimental and simulation results show that the multipoint voltage monitoring method can avoid the ‘voltage cheat’ and therefore achieve the real consistency and uniformity performance.

Evaluation of Two Water Transports through Electrolyte Membrane of Polymer Electrolyte Fuel Cell Based on Water Visualization and Current Measurement

AbstractTo alleviate membrane dehydration in polymer electrolyte fuel cells (PEFCs) during low‐humidity operations, it is essential to deeply understand two water crossover phenomena through electrolyte membranes. This study presented the quantitative method for separately evaluating the electro‐osmotic and back‐diffusion water transports in the electrolyte of an operating PEFC based on the measurements of water vapor and current distributions. Firstly, the water vapor distribution in the anode flow field was investigated by the visualization technique using humidity test paper (HTP). Subsequently, the detailed transport processes of water through the electrolyte membrane can be numerically estimated from the measurement results of water and current profiles by using a simple fuel cell model. In this work, the effects of inlet gas humidification, microporous layer (MPL) addition and flow configuration on the two water transports in the membrane were demonstrated under low‐humidity conditions to verify the validity of the proposed methodology. Results revealed that the introduction of MPL on the cathode side increases the water concentration in the anode channel due to the back‐diffusion effect. The counter‐flow configuration effectively improves the membrane hydration at the anode inlet because of the strong back‐diffusion from the wetted cathode outlet.

In situ durability of various carbon supports against carbon corrosion during fuel starvation in a PEM fuel cell cathode

In this study, the degradation of different cathode carbon supports is investigated in proton exchange membrane fuel cells (PEMFCs). A platinum catalyst is synthesized using various carbon supports, such as Vulcan XC-72, graphite nanopowder and carbon nanotube, which are evaluated based on the fabrication of membrane electrode assemblies. During the startup and shutdown of PEMFCs, the individual electrode potential can be measured in situ using a dynamic hydrogen electrode. The cathode potential increases instantaneously to 1.4 V in one attempt, when H2/air boundaries are developed on the anode side during the fuel starvation, leading to significant carbon corrosion. The corrosion rates of various carbon supports are calculated from the concentration of gases, such as CO2, CO and SO2, emitted from the cathode outlet, measured directly in situ by Fourier transform infrared gas analysis. The carbon nanotube-supported Pt catalyst shows the best performance against carbon corrosion during fuel starvation, compared to commercial Pt/C catalyst and other types of carbon supports.

A locally resolved investigation on direct methanol fuel cell uneven components fading: Local cathode catalyst layer tuning for homogeneous operation and reduced degradation rate

Durability issues of direct methanol fuel cell still hinder technology widespread commercialization; uneven aging of MEA components, generally harsher in air outlet region, is known to exasperate overall performance degradation. In a previous work, the authors selected a stable cathode electrode, demonstrated to fade homogenously: uneven water-related limitations, such as dehydration and flooding, were revealed to locally worsen performance at cathode inlet and outlet regions, leading to current redistribution. Aiming to reduce degradation rate, in this work homogeneous current distribution during operation is pursued by tuning MEA properties to meet local operating conditions. A properly improved 1D+1D physical model is used to support the development of a gradient MEA, featuring 1.6 mg cm−2 and 0.8 mg cm−2 of catalyst and ionomer respectively at inlet/outlet and center regions of cathode electrode. Tests based on custom macro-segmented cell demonstrated 55% more homogeneous current distribution, controllable during operation by means of cathode air stoichiometry. 500 h degradation test revealed 70% decreased degradation rate from uniform MEA (11 μV h−1) with a homogenous fading of performance. An 18% lower Pt nanoparticle growth at cathode outlet and limited ionomer degradation at cathode inlet were identified by ex-situ analyses (TEM and XPS), indicating locally mitigated fading mechanisms.

Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover

A parametric study was carried out to investigate the effect of fuel concentration (0.5 M–3.0 M), operating temperature (ambient temperature to 85 °C), flow rate of ethanol (0.5–5.0 mL min−1) and air (100–600 mL min−1) on the direct ethanol fuel cell (DEFC) performance. The operations were conducted in three operational modes, namely, passive, semi passive, and active modes, and power generation were measured. Ethanol crossover was indicated by the carbon dioxide (CO2) concentration present at the cathode outlet and measured by using a CO2 analyzer. Results indicated that DEFC performance increased with the increase of ethanol concentration, and ethanol and oxidant flow rate increased with temperature until DEFC reaches the optimum conditions, i.e., concentration and flow rate. Meanwhile, the DEFC performance significantly and proportionally increased with operation temperature and reached values of up to 8.70 mW cm−2 and 85 °C at stable conditions. Furthermore, fuel crossover, that is, ethanol flux, increased in proportion to the ethanol concentration, i.e., 3.71 × 10−4 g m−2 s−1 and 8.79 × 10−4 g m−2 s−1 for 0.5 M and 3.0 M ethanol concentration, respectively. At different modes of operation, the active DEFC system exhibited the highest performance, followed by the semi passive and passive DEFC system. These results indicated that optimizing ethanol, oxidant flow rate and temperature would enhance the mass transport in anodes and cathodes, and hence improve the electrochemical reactions and DEFC performance.

A Comparison of the Proton Exchange Membrane Fuel Cell Water Balance Measured Simultaneously with the Water Knock-out Method and Hot Wire Anemometry

The water balance of a proton exchange membrane fuel cell (PEMFC) can provide valuable insight into the state of the fuel cell and may even limit the current density due to anode dry-out. Typically, commercial fuel cell test stations can be equipped with a method of detecting the fuel cell water balance using the water knock-out method. In this method, the anode and/or cathode exhaust gas streams are led through a condensing unit and the liquid water that is collected over time is back-calculated in terms of the water balance, usually denoted rd and defined below. Thus, it is a dimensionless property that indicates how much product water has been transported through the membrane from anode to cathode, and negative values indicate overall back-transport from cathode to anode. The definition is also such that when multiplying with the factor 2 one obtains the percentage of the product water that has crossed the membrane, i.e. a value of -0.02 indicates that 4% of the product water has been back-transported from cathode to anode. Adding the option to measure the water balance to fuel cell test stations increases the cost significantly because additional equipment has to be added to the system such as heat exchangers, through-liquid drainers, beakers, mass scales, control valves and all the hardware and software for controlling and integrating the water knock-out system. After the gas stream escapes from the cathode and anode outlet it passes through the heat exchanger in order to cool the gas stream and condense the water in the stream. The liquid drainer delivers the product water to a beaker on a mass balance scale. The software monitors the mass until the water level reaches a high point, and then drains the water automatically through a solenoid (GREENLIGHT INNOVATION, Canada). Because there is no external cooling reservoir, the condensing of water occurs at room temperature and some water vapor escapes the measurement. In addition, such an experiment has to run for an extended period of time in order to collect a sufficient amount of water for an accurate result. Meanwhile, our research group has developed a novel method to measure the fuel cell water balance directly and in real time using hot wire anemometry. This method yields a voltage signal that can be directly converted into the fuel cell water balance such that the fuel cell water balance can be a direct output signal similar to voltage and current density. Because, this method requires the hot wire and a heating coil to control the temperature as an extra added hardware in the anode outlet. In addition, it provides an ad-hoc real time electrical signal of the PEMFC water balance, thus the cell size and the current density have no significant effect on the accuracy of measured water balance. The Ad-hoc real time rd measurements allow this method to be part of FCV systems and/or their test equipment. Behind the test section the anode exhaust gas is just led through the fume hood and o drainage equipment is required.In this work, the water balance for 5 KW water-cooled PEMFC stack is simultaneously measured using the two aforementioned methods. The stack is run under anode and cathode wet conditions at different current densities and stoichiometric ratios. The hot-wire method has shown a higher sensitivity and an instant response for the measured rd when changing the running conditions such as the current density. Meanwhile, the knock-out method showed less sensitivity and much slower time response for the measured rd. In addition, it suffers from higher inaccuracies due the loss water vapor with the exhausted gases. However, Figure 1 shows that the overall comparison between both methods is very good and may serve as validation of the hot wire method. Figure 1

Study on voltage clamping and self-humidification effects of pem fuel cell system with dual recirculation based on orthogonal test method

Durability and reliability are still major challenges of vehicular polymer electrolyte membrane fuel cell (PEMFC) systems. With exhaust gas recirculation on both the anode and cathode sides, two important functions can be achieved: the voltage clamping in low current density, and the self-humidification without any external humidifiers. The former restrains catalyst decay in small load working conditions, and the latter is beneficial for improving the cold-start ability. In this study, dynamic performances and stable characteristics of a fuel cell system with dual exhaust gas recirculation are firstly experimentally studied using an orthogonal test method. System parameters, including humidification temperature of cathode external humidifier, fresh air stoichiometric ratio (SR), current density, cathode and anode recirculation pump speeds, are regarded as key factors in the experiments based on the testing conditions of the test-bench. Two four-factor and three-level orthogonal tables are designed, and the effects of key factors on system performance indices (average cell voltage, relative humidity (RH) at cathode inlet, high frequency resistance (HFR), oxygen concentrations at cathode inlet and outlet, and the concentration difference between these two positions) are investigated. Results show that: (1) with the cathode recirculation, the cell voltage can be reduced in low current densities by coordinately adjusting the recycled gas flow and reducing fresh air SR; (2) with the dual recirculation, the fuel cell membrane can be well hydrated, and system performance only shows 3% reduction compared with a system with an external humidifier; (3) the difference between the oxygen molar concentration at the inlet and outlet of cathode gas channels becomes small using dual recirculation.

A locally resolved investigation on direct methanol fuel cell uneven components fading: Steady state and degradation local analysis

DMFC technology widespread commercialization is still hindered by durability issues. In literature, locally resolved measurements and post-mortem analyses reveal the onset of strong heterogeneous components fading, higher at air outlet region. Local aging mechanisms could be enhanced by DMFC cathode cycling operation in highly uneven presence of water. An innovative PEM macro-Segmented Fuel Cell (mSFC) setup has been developed to investigate DMFC local performance and degradation, coupling electrochemical and ex-situ analyses as TEM and XPS. An MEA based on highly graphitized carbon supported cathode electrode, confirmed in AST to be stable to cycling operation under flooded conditions, has been implemented in DMFC operation. 500 h local degradation test, despite 32% heterogenenous current density distribution, reveals homogeneous ECSA loss, consistent with homogeneous nanoparticles growth from 3.16 to 5.4 nm, which leads to 70% lower degradation rate (31.2 μV h−1) than the reference MEA. Water-related limitations, such as dehydration and flooding, are revealed to increase local performance loss by 25% and 100% respectively at cathode inlet and outlet regions, leading to current redistribution and uneven voltage loss. Hence, local optimization of MEA properties is foreseen to permit further important durability improvements.

Effect of iodine precipitation on HI separation subsection in sulfur-iodine cycle for hydrogen production

As a critical subsection in the sulfur-iodine (SI) thermochemical cycle, HI concentration and separation must cope with the pseudo-azeotropy of HIx (HI-I2-H2O) and excess iodine in HIx solution. Although electro-electrodialysis (EED) coupled with conventional distillation is a validated method of HI separation from HIx solution in the SI process, the iodine deposition, resulting from changes in temperature and HI molality in HIx solution, can lead clogged flow channels of the EED anode and other tubes. A precipitator can address this problem by recovering excess iodine from HIx solution after the HIx purification column. The energy duty and required input flow rate per mole HI was investigated in this study using a process flowsheet simulation. A decrease in iodine concentration in the streams to EED could reduce cell duty effectively. An increase in HI molality in the EED cathode outlet resulted in an increase in EED duty; however, the amplitude was slight. The iodine molar concentration in the feed of the distillation column exhibited an appreciable influence on the distillation duty. However, with an increase in distillation column pressure, the effect of diminished iodine in feed on the HI distillate duty continued to decline. To assess the utilization of an iodine precipitator in the HI separation subsection, the energy demands and required input flow rates of three different flowsheets were calculated using Aspen Plus and Microsoft Office Excel. Results indicated that the flowsheet that only recovered iodine in the stream to the EED anode chamber exhibited the least HI separation duty and the lowest required input flow rate.

Investigation of Degradation Heterogeneities in PEMFC Stack Aged under Reformate Coupling In Situ Diagnosis, Post-Mortem Ex Situ Analyses and Multi-Physic Simulations

Proton Exchange Membrane Fuel cells (PEMFCs) durability of stacks operated under reformate is investigated with a special focus on the heterogeneity of aging. During an aging test at constant load, the local performances were investigated in situ using a segmented circuit board and a specific CO poisoning diagnostic tool based on the transition from pure hydrogen to reformate containing carbon monoxide. The heterogeneities analyses are supported with multi-physic simulations which highlight the heterogeneous CO coverage along the anode, as well as the competition between both electrodes leading to non-monotonous current density profiles. At the end of life, electrochemical and transmission electron microscopy analyses were performed on three characteristic zones (air inlet/H2 outlet, middle and air outlet/H2 inlet) of the Membrane Electrode Assembly (MEA). These experimental investigations put in evidence that the cathode outlet aged more severely than the cathode inlet, while more CO tolerance was lost at the anode outlet. The degradation by the electrochemical Ostwald ripening mechanism of the Pt3Co nanoparticles at the cathode outlet is suspected to pollute the ionomer, leading to the observed accelerating performance losses. Finally, optimized MEAs have been designed to mitigate the suspected degradation mechanisms, and tested at stack level demonstrating a clear durability improvement.

Performance degradation of a proton exchange membrane fuel cell with dead-ended cathode and anode

Proton exchange membrane fuel cells with dead-ended cathodes and anodes can simplify the fuel cell system and reduce costs. An experiment was performed to determine the performance degradation characteristics of Proton exchange membrane fuel cells with dead-ended cathodes and anodes. The effects of operating temperature and pressure differences between the cathode and the anode on the purging period were investigated in detail. The performance and cyclic voltammetry before and after the dead-ended operation were analyzed and compared. After the experiment, the membrane electrode assembly was cut to analyze the catalytic layer cross-section membrane morphology by scanning electron microscopy. The results showed that during operation, the fuel cell performance gradually decreases until the setting value, and then quickly recovers when the cathode outlet solenoid valve is triggered during a purging cycle. The dead-ended operating period decreases with an increase in operating temperature but increases with an increase in the pressure difference between the cathode and the anode. Flooding occurs easily in a Proton exchange membrane fuel cell with a dead-ended cathode and anode, causing performance degradation. Moreover, it may cause a decrease in the electrochemical surface area of the catalyst layer. The scanning electron microscopy images showed that both the upper and middle regions of the catalyst layers remained unchanged, whereas the downstream region corroded and become thinner in the dead-ended mode after 60 h.

Experimental measurement technique for the assessment of the fuel crossover diffusion coefficient in the membrane electrode assembly of a direct methanol fuel cell

Since the cross-over still seems to be the main issue of the direct methanol fuel cells, an experimental evaluation of the diffusive cross-over is performed. Even if the relationship of the rate through the membrane is the sum of the three terms of diffusive, osmotic and drag, the diffusive component is also present at open circuit lowering the Open Circuit Voltage of the single cell up to 50 % with respect to the Nernst potential. The goal of the research is to develop a direct measurement technique of the crossover that can provide the effective values of the parameters that characterize the membrane electrode assembly. The experimental set up consists in the pressure, flow and temperature control and acquisition using Labview. A sensitive analysis for three values of temperatures at 60°C, 65°C and 70°C is performed for first. Then, a small overpressure was generated in the cathode side by a valve located at the cathode outlet. A set of pressure were analysed for 0, 30 and 90 mbar of overpressure at the cathode. The tested fuel cell has a commercial Nafion 117 membrane and carbon paper gas diffusion layers 700 cm2 large. Preliminary results show that the differential concentration term seems to be significantly larger than the osmotic term. The diffusion coefficients are useful for fuel cell modelling and for the calibration of the operating conditions in the sensor less DMFC systems.

New design of a PEFC cathode separator of for water management

Generally, polymer electrolyte fuel cells (PEFCs) need humidifiers to prevent the drying of the membrane, but this use of humidifiers creates water management issues, such as the flooding/plugging phenomena and decreased system efficiency because of an increase in the electric energy needed for auxiliary equipment. Although most researchers have developed high-temperature membranes that do not need humidifiers, a lot of time is necessary for the development of these membranes, and these membranes drive up costs. Therefore, we propose a new cathode separator design that can recycle water generated by power generation in the same cell and a stack structure that can redistribute water collected in the cathode outlet manifold to drying cells. Because the new cathode separator has a bypass channel from the gas outlet to the gas inlet to transport excess water, a dry part in the gas inlet is supplied with excess water in the gas outlet through the bypass channel even if the PEFC is operated under dry conditions. Excess water in the PEFC stack can be transported from the cell with excess water to the drying cell through the cathode outlet manifold with a porous wall. Therefore, we confirm the influence of the plugging phenomenon in the cathode gas outlet manifold on the cell performance of each cell in the stack. As a result, the cell performance of the new cathode separator design is better than that of the standard separator under the low humidity conditions. We confirm that the plugging phenomenon in the cathode outlet manifold affects the cell performance of each cell in the stack.

Characterization of High Temperature PEM Fuel Cells for Electrochemical Hydrogen Pumping

High temperature proton exchange membrane fuel cells (HT-PEMFCs) based on phosphoric acid doped polybenzimidazole operate at a temperature range between 140°C and 200°C. Due to their high operating temperature, HT-PEMFCs can tolerate a relatively high concentration of impurities in the anodic fuel stream. This makes HT-PEMFCs suitable for stationary applications where hydrogen is generated on site by steam reforming of hydrocarbons such as methane or oil derivatives. In particular, the preferential oxidation stage (PROX), which is typically required to reduce the CO concentration to ≈10 ppm after the reforming and water gas shift (WGS) reactors, is no longer necessary. Thus, fuel treatment units are simpler and cheaper with HT-PEMFCs [1]. The proven robustness of HT-PEMFCs against fuel impurities has recently attracted interest to operate the HT-PEMFC as a ‘hydrogen pump’ for hydrogen purification applications. In this configuration, a hydrogen rich gas stream is supplied to the cell’s anode compartment and a current is externally applied through the cell. Different from a typical fuel cell, no oxygen is supplied on the cathode side. During this process, hydrogen is electrochemically ‘pumped’ from the anode to the cathode in the form of protons through the electrolyte membrane. The ‘pump effect’ is highly selective and a pure hydrogen stream is obtained at the cathode outlet of the cell (≈10 ppm). Compared with low temperature Nafion based fuel cells, HT-PEMFCs are much more promising for purification of hydrogen streams that contain considerable amounts of impurities, as proven by preliminary studies [2, 3]. In this work, an investigation of the HT-PEMFC operated as an electrochemical hydrogen pump is carried out. The impact of hydrogen dilution and CO poisoning is studied as well as the effects of hydrogen stoichiometry, gas humidification, and electrolyte resistance. The cell is characterized with various techniques including polarization curves, electrochemical impedance spectra (EIS), and distribution of relaxation times (DRT) analysis (Fig. 1). Complementary to the equivalent circuit approach, DRT maps the EIS data to a function that contains the characteristic time scales of the system [4]. These results are important for the assessment of the HT-PEMFC’s potential for hydrogen purification. Although this application is distinctly different from that of a typical fuel cell, the electrochemical process at the anode is actually the same as in a normal HT-PEMFC. Therefore, the study also helps us gain valuable insight into the hydrogen oxidation reaction process in the presence of impurities.

Modeling of Methanol-Fueled SOFC with Anode Offgas Recycling for Simplified System Design

Objective :Methanol is a fuel that can be synthesized from either fossil fuel or biomass. Use of methanol as a fuel in SOFCs offers several advantages. A major advantage lies in its ease of storage, handling, distribution, and input, being found in liquid form under ambient conditions. Compared to most other fuels, methanol is rather easily produced, and is known to have a lesser tendency for carbon deposition when vaporized and heated at high temperature.In this work, we conducted modeling and simulations of a methanol-fed SOFC system with anode gas recycling. Anode gas recycling provides water vapor in the fuel inlet, thus decreasing the tendency of the feed gas to form carbon deposits in the fuel cell stacks. This strategy also helps reduce thermal stresses in the stacks that would arise if the reforming was attempted in-situ with 100% water vapor originating from outside the SOFC system. The recycling also helps increase the fuel utilization. A thermodynamic analysis was conducted to calculate the energy generated, absorbed, or exchanged in the various blocks of the system, the expected system energy output, as well as conditions under which carbon deposition is prevented. Results:The model is based on an integrated energy process with anode gas recycling, a rated output electrical power of 100 kW, and an electrical efficiency of 50% to initiate simulations. Using the CHEMCAD software, equilibrium gaseous compositions were calculated. The oxygen flow rate and the cell temperature were set at five times the stoichiometric value and 800°C, respectively. Thermodynamic calculations show that the gas mixture at the anode inlet will not lead to carbon deposition above 575°C. The energy generated in the cell by electrooxidation is calculated at 144.6 kW. 44.6 kW of thermal energy is thus generated at the stack level. This heat is removed by the anode and cathode outlet flows, as well as minor heat losses through the system walls. A mass recycle ratio of 1.47 was obtained. Energy involved in each of 9 system blocks was calculated. Overall system and electrical efficiencies of 75% and 57% were obtained from simulations based on lower heating value. A voltage ranging from 0.91 to 0.94 V across the cell stacks was obtained at a fuel utilization factor of 80% and a temperature of 800°C. Finally, simulation iterations lead to an output fuel cell power of 98.8 kW, which is close to the power value used as input at the start of simulations.Conclusion:The recycle ratio of 1.47 contributes to system efficiency. By contrast, it was reported that for propane, a fuel utilized in Northern Canada, a recycle ratio greater than 5 is required for reliable functioning of SOFC systems. Modeling and simulation results clearly showed that methanol has very little tendency to form carbon deposits during cracking in situ when used in a simple anode gas recycling system. A very good electrical efficiency is thus obtained.

Investigation of Solid Oxide Electrolysis Cell Electrodes for Methane Synthesis

Electrochemical methane synthesis from CO2 and water is one of the means of storing intermittent renewable energy. It also attracts attentions because it can contribute to the fuel cycle which does not rely on fossil sources. Methane-synthesis solid oxide electrolysis cells (MS-SOEC) can be a candidate technology for such processes. The concept is shown in Figure 1. A novel point of this concept is that CO methanation reaction is integrated into well-known co-electrolysis. The methanation is favoured at relatively low temperature (below 800oC) from the equilibrium viewpoint. It is reported that exothermic nature of the reaction leads to smaller heat loss when combined with endothermic electrolysis, which results in higher round-trip efficiency for energy storage[1,2]. However, the number of experimental researches on MS-SOEC is very limited, and investigation of suitable materials for MS-SOEC is necessary to realize this concept. In this study, several electrodes were tested for MS-SOEC application. Because anode polarization resistance is dominant in SOEC operation[3], anode materials play an important role. Perovskite-type oxide-based materials are commonly used for SOEC anodes. Among them, LSM (La0.6Sr0.4MnO3-δ) and LSCF-SDC composite (LSCF: La0.6Sr0.4Co0.2Fe0.8O3-δ, SDC: Ce0.8Sm0.2O1.9, LSCF 50wt%) were compared. Electrolyte and cathode were fixed in YSZ (8mol% yttria stabilized zirconia) and Ni-SDC composite (Ni 60wt%), respectively. Electrode pastes were screen-printed on both sides of YSZ electrolyte discs and calcined for 5 hours at designated temperatures. For LSCF-SDC anode, an interlayer of SDC was placed between the electrolyte and the anode using spin coating technique to prevent them to react. Before measurements, NiO in the cathode composite was reduced by hydrogen flow at 1000oC. In MS-SOEC test, 30 mL min-1 air was fed to the anode side while cathode feed gas was CO2(10)/H2O(5)/H2(10)/N2(7) mixture. Figures in parentheses indicate gas flow rate in mL min-1. Hydrogen was added to the cathode gas to stabilize the cell performance. This is a possible condition in industrial operations, because part of the cathode outlet gas can be recycled and mixed into feed stream. Cell operating temperature was set to 800oC. AC impedance measurements were conducted for frequency range of 106~0.1 Hz with amplitude of 10 mV RMS. Current-voltage characteristics were also measured. Cathode outlet gas composition was checked by micro gas chromatograph for open circuit and 1.0 A cm-2 galvanostatic conditions. With the aim of improving cathode performance, typical cermet of Ni-YSZ was modified by ruthenium impregnation. Ruthenium is expected to suppress nickel oxidation which is likely to occur in oxidizing atmosphere, and also has high activity for methanation reaction. Therefore, it is a promising candidate for MS-SOEC cathode additives. Ruthenium nitrate solution was dropped on calcined cathode of NiO-YSZ|YSZ|LSM cell and dried at room temperature. Ruthenium amount was set to be 5% of cathode weight. Then the cell was kept in nitrogen atmosphere during temperature rising, and at 1000oC, ruthenium precursor was reduced along with NiO by hydrogen flow. AC impedance spectra for cathode were taken in three-electrode configuration at 800 and 600oC, then the data were analyzed by introducing Δ∂Z' spectra[4]. In this experiment, gas conditions were the same as the previous tests. In all tests, progression of co-electrolysis and methanation were confirmed by gas analysis. According to AC impedance spectra of Ni-SDC|YSZ|LSM and Ni-SDC|YSZ|SDC|LSCF-SDC cells, significantly small non-ohmic resistance was observed with LSCF-SDC anode. Galvanostatic test results in Figure 2 indicate that cell performance was more stable with LSCF-SDC. Impedance data of the cells with or without ruthenium were compared by calculating Δ∂Z' value. The resultant Δ∂Z' spectra showed peaks around 1~10 Hz. This implies that electrochemical activity was enhanced by ruthenium addition.

Study on Nonlinear Identification SOFC Temperature Model Based on Particle Swarm Optimization–Least-Squares Support Vector Regression

In order to facilitate valid solid oxide fuel cell (SOFC) temperature control scheme, a nonlinear identification method of SOFC temperature dynamic behaviors is proposed using an autoregressive network with exogenous inputs (NARX) model, whose nonlinear function is described by a least-squares support vector regression (LSSVR) method with radial basis kernel function (RBF). During the identifying process, a particle swarm optimization (PSO) algorithm is introduced to optimize the parameters of LSSVR. On the other hand, a mechanism model is developed to sample the training data to regress the NARX model. Investigations are conducted to analyze the effects of training data size and PSO fitness function on the accuracy of the NARX model. The results demonstrate that the NARX model with tenfold cross-validation fitness function and large size data is precise enough in predicting the SOFC temperature dynamic behaviors. The maximum errors of cathode and anode outlet temperature are 0.3081 K and 0.3293 K, respectively. Furthermore, the simulation speed of NARX model is much faster than the mechanism model because NARX model avoids the internal complex computation process. The training time of the NARX model with large size data is about 1.2 s. For a 20,000 s simulation, the predicting time of the NARX model is about 0.2 s, while the mechanism model is about 36 s. In consideration of its high computational speed and accuracy, NARX model is a powerful candidate for valid multivariable model predictive control (MPC) schemes.

Modeling and analysis of internal water transfer behavior of PEM fuel cell of large surface area

The PEM fuel cell has been widely used in the area of transportation and power station. The surface area of a fuel cell is enlarged to provide high enough power but the problem of analysis of internal water content behavior follows tightly. Many scholars have investigated the mathematical models of a small fuel cell and validated them through experiment. Besides, the introduction of AC impedance technique helps find relationship between water content and membrane resistance. Based on their research, an approach is put forward in this paper to model and analyze the internal water content behavior in a fuel cell of large surface area. For large surface area, three special cases are studied according to the actual operating states at cathode outlet. The first case applies to a fuel cell with no saturated water vapor at both outlets while in the second and third case, the fuel cell is divided into an electrochemical reaction zone and no reaction zone owing to emerging liquid water. The indicators of model are the water content profile inside membrane and the total membrane resistance. The simulation results show that the net water transfer coefficient has significant influence on the performance of the membrane and the constituents of anode side are easy to be varied. In addition, when the fuel cell is operated in counter-flow mode with emerging liquid water, the only back diffusion of water from cathode to anode helps improve the state of the membrane.

Investigation of Solid Oxide Electrolysis Cell Electrodes for Methane Synthesis

Methane-synthesis solid oxide electrolysis cell (MS-SOEC) technology has potential to provide efficient renewable energy storage systems. However, there are few studies focusing on MS-SOEC electrode materials. In this study, several materials were tested experimentally. As anode materials, La0.6Sr0.4MnO3-δ (LSM), La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.8Sm0.2O1.9 composite (LSCF-SDC) and platinum were compared. The cell with LSCF-SDC exhibited lower overpotentials and higher stability than LSM. Platinum showed even better performance than LSCF-SDC. Ni-SDC was used for the cathode, and the cathode outlet gas compositions were analyzed for several operation temperatures and current densities. It was found that catalytic activity for methanation was enhanced under polarized conditions. Effects of ruthenium addition to the cathode were also examined. Impedance analysis combined with gas composition measurements revealed high electrochemical activity of ruthenium, while methane production was suppressed possibly due to methane reforming reactions promoted by ruthenium.